Vertical Crustal Movements Flashcards
Contractional (Compressional) Regime
Stress Type: Compression
Plate Boundaries: Convergent boundaries
Result: Crustal shortening, thickening, and deformation
Main Structural Features:
1) Folds- Anticlines (upward arching folds) and synclines (downward arching folds).
2) Thrust Faults/Reverse Faults- High-angle (reverse) or low-angle (thrust) faults where older rocks are pushed over younger rocks.
-Results in crustal thickening.
3) Metamorphic Foliation- Development of schistosity or gneissic banding due to intense pressure.
Extensional Regime
Stress Type: Tension
Plate Boundaries: Divergent boundaries
Result: Crustal thinning and fracturing
Main Structural Features:
1) Normal Faults- Faults where the hanging wall moves down relative to the footwall.
-Associated with crustal extension and thinning.
Horsts and Grabens:
2) Alternating uplifted blocks (horsts) and down-dropped blocks (grabens).
-Common in rift valleys and extensional basins.
3) Fault-Block Mountains- Created by large-scale block faulting during extension.
4) Basin and Range Topography- A characteristic landscape of alternating mountains and valleys formed by extension.
Conservative (Transform) Regime
Stress Type: Shear
Plate Boundaries: Transform boundaries
Result: Horizontal displacement, minimal crustal creation or destruction
Main Structural Features:
1) Strike-Slip Faults- Faults where two blocks slide past each other horizontally.
2) Fault Zones and Damage Zones- Extensive zones of fractured rock surrounding major strike-slip faults.
Why do Topography and Bathymetry Reflect an Out-of-Equilibrium Earth’s Surface.
The Earth’s surface, both on land (topography) and beneath the oceans (bathymetry), is constantly changing due to erosion, sedimentation, and mantle convection. These processes create landscapes that are out of equilibrium, meaning that the surface is continually evolving toward (but rarely reaching) a state of balance.
What is load pressure?
Stress caused by the weight of overlying rock.
What is deviatoric stress?
Stress which is not uniform and is caused by lateral transmission of horizontal tectonic forces.
What are the 3 principle perpendicular stresses?
Sigma 1= Max principle stress
Sigma 2= Intermediate principle stress
Sigma 3= Min principle stress
What are the orientations of sigma 1 and 3 during compressional settings?
Sigma 1 is horizontal and sigma 3 is vertical.
What are the orientations of sigma 1 and 3 during extensional settings?
Sigma 1 is vertical and sigma 3 is horizontal.
What are the orientations of sigma 1 and 3 during shear settings?
Both sigma 1 and 3 are horizontal.
What factors control whether a material is brittle or ductile?
- Material properties
- The load/ Lithostatic stress
- Temperature (higher temp=more ductile.)
- Strain rate (quicker rate of stress=more likely to behave in a brittle manner.)
What are faults?
Faults are where movement has occurred along a dipping surface and the two blocks have moved with a vertical component relative to the surface.
Normal faults
-Hanging wall is downthrown and footwall upthrown.
-Thins the rock (extensional.)
-Steep (60 degrees from the horizontal.)
Thrust or reverse faults
-Hanging wall is upthrown
-Thickens the crust (compressional.)
-Low angle (30 degrees from the horizontal.)
-Associated with folds.
Strike-slip faults
-No hanging walls.
-Vertical fault.
-Shearing.
-No net change in crustal thickness.
What caused most folding?
Buckling (compression resulting in plastic deformation.)
Why is true equilibrium never reached on the Earths surface?
- Time Scales Mismatch:
-Tectonic processes operate over millions of years, while erosion and sedimentation act on much shorter time scales. This mismatch means that the Earth’s surface is always in a state of “catching up” to the forces acting upon it.
- Feedback Loops and Delayed Responses:
-The interplay between uplift, erosion, sediment deposition, and isostatic adjustment creates delayed responses that keep the system in motion.
What are the three main mechanisms for vertical crustal movement?
- Isostasy
- Flexural loading
- Dynamic effects
What is Isostacy?
Isostasy refers to the balance between the Earth’s lithosphere and asthenosphere. When the weight or density of the crust changes, it responds by rising or sinking to maintain gravitational balance.
Similar to how a floating object displaces water, the lithosphere “floats” on the denser, ductile asthenosphere. Any change in mass distribution causes the lithosphere to adjust to maintain equilibrium.
What is flexural loading?
Changes in load on the lithosphere over 100km in wavelength are compensated for by bending of the Earth’s lithosphere. When a force, such as the weight of mountains, sediments, or ice sheets, is applied to the lithosphere, it bends or flexes. This bending results in subsidence beneath the load and uplift at the margins, creating a characteristic pattern of deformation.
What is dynamic effects?
Dynamic effects within the asthenosphere such as upwelling or sinking of material causes flexure of the lithosphere.
What does thinned crust lead to?
-Basin formation (airy isostacy.)
-High heat flow.
-Volcanic activity.
What are the two types if extension?
Passive rifting and active rifting.
Passive rifting
Regional extension causes rifting which then causes decompression melting and upwelling. (Basin and range, USA.)
Active rifting
Hot mantle plumes cause uplift which then causes extension and collapse under gravity. (East African rift valley.)
What are failed rifts?
Basins floored by oceanic crust, but where extensional strain has not been maintained and the geothermal gradient has returned to normal.
Describe the vertical crustal movements and subduction settings?
Partial melting removes material from the subducting slab and adds it to the overriding slab. An accretionary prism forms where scraped sediment from the subducting slab builds up. This all results in crustal thickening and uplift due to isostacy.
The angle of subduction steepens with time which leads to extension and subsidence behind the arc. (Japan)
Describe the vertical crustal movements and continental collision boundaries?
One plate is forced under the other resulting in lots of deformation= thickening and uplift (isostacy.)
What are foreland basins?
Increased load due to thickening flexes the crust downward either side of a mountain belt forming foreland basins.
